Certificate authority (CA) A certificate authority is an entity similar to a notary public. It issues digital certificates, signs certificates to verify their validity and tracks which certificates have been revoked or have expired. Top of Page Digital signature assurances The following terms and definitions show what assurances are provided by digital signatures. Authenticity The signer is ...

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About digital signatures and certificates in Office, and buying a digital certificate from a certificate authority or create your own.

See how to view the details for signatures and certificates.

Contact your admin if you need help. Support options are available to admins who can sign in to the Microsoft 365 admin center with their admin account to contact support on behalf of users in their organization. If your product is listed below, you have a business product.

Don't forget to include your license ID or agreement number, your contact phone number & email address so we can contact you. If you already have a volume licensing support case number, you may be in a location where you can phone the Volume Licensing support team.

Learn how to activate Office that is pre-installed on new Windows 10 PCs or using a new product key.

When you enter a recipient's name in the Bcc (blind carbon copy) field of an email, that recipient receives a copy of the email without their name being visible to the other recipients. Select a tab option below for the version of Outlook you're using. What version of Outlook do I have?

Product key, purchase, and installing older versions of Microsoft 365: Install questions or errors during install Help with specific applications, and help for Microsoft 365 admins: Contact Microsoft

Describes how to use the System File Checker tool to troubleshoot missing or corrupted system files in Windows.